
The first term in (REF ) is the energy of the transverse and longitudinal fields and (REF ) is the energy of the “interference”\(\,\)  between the electric transversal modes and the magnetic transversal modes. We observe that the transversal electric and the transversal magnetic fields have the same energy, but that there is an interference term, that in general, will be different from zero, as the author of Reference [1]} noted studying the Poynting vector. It is worth to note, that the expression reported here, is valid for any invariant field and that the time averaged energy density does not depend on the longitudinal variable \(z\) , as must be.
